enable local delivery for nodes using the MTA as relayhost

currently the default mail setting for our nodes is to not have any local destination domain and relay everything to our MTA. this has the downside that if mail is sent from and to a non-existing address, it disappears into the void. this is actively happening on several nodes, mostly cronjob outputs from users that don't have an alias defined:

• a cronjob runs on a node as a specific user
• the node creates a mail from user@node.torproject.org to user@node.torproject.org
• the mail is sent to mta-dal-01.torproject.org
• the mail is then sent on to mx-dal-01.torproject.org
• mx-dal-01 doesn't know about user, wants to bounce to user@node.torproject.org but also doesn't know where to send that bounce and drops the mail

i'd like to propose adding the FQDN as local destination domain on all nodes. that way such mail will end up delivered in /var/mail, prompting an alert, and coercing us to create an alias.